Justicia extensa is an ichthyotoxic plant extensively used to stun fish by artisanal fisher folks in south-south Nigeria. The effect of aqueous extract of this plant leaf was investigated on behavior and histology of juvenile Clarias gariepinus. The plant leaf extract was administered to the fish specimens at different acute doses (0, 25, 35, 45, and 60 mg/L) for 96 hours. During this time, the grouped animals’ behavioral and histological (liver, gill) changes were evaluated. Based on our findings, the 96-hour LC50 value of 46.59 mg/L was obtained. The study also showed that, with the exception of temperature, the plant influenced the test media’s water quality parameters in a concentration-dependent manner. After 96 hours of exposure, J. extensa caused gill (necrosis, hyperplasia and hypertrophy of secondary lamella, mucous secretion, cartilaginous core and cellular degeneration) and liver (central vein congestion, coagulation necrosis, sinusoidal spaces dilation and inflammatory cells infiltration) histological abnormalities as well as noticeable behavioral aberrations (air gulping, abnormal opercula ventilation, loss of equilibrium, erratic swimming, sluggish movement and change in skin color) in C. gariepinus . Therefore, caution should be applied in the use of J. extensa to avert possible disaster in the aquatic ecosystem.